First experience with the use of direct monitoring of the auditory nerve in vestibular schwannoma surgery in the Czech Republic
Abstract
Introduction: Hearing preservation during vestibular schwannoma surgery is still a challenge even for experienced surgeons. Monitoring of evoked brainstem potentials (brainstem evoked response audiometry; BERA) is one of the standard methods of perioperative hearing evaluation; however, in recent decades, its potential to further improve hearing preservation outcomes has been fully exhausted.
Monitoring of direct responses from the cochlear nerve (cochlear nerve action potential; CNAP) is not a widespread technique so far. Direct monitoring informs surgeons almost immediately about the condition of the inner ear and auditory nerve, allowing them to react quickly by changing surgical techniques.
Materials and methods: During the February-May 2021 period, we used direct monitoring in 4 patients. An AVALANCHE(R)XT monitoring unit (Dr.
Langer Medical GmbH, Waldkirch, Germany) was used for monitoring, allowing simultaneous monitoring of BERA, CNAP, and the facial nerve. In three patients, the primary goal was to preserve hearing.
In the fourth patient with a large tumor, the goal was to track the auditory nerve. Results: Hearing has been preserved in two patients postoperatively.
In the first case, both evoked potentials and direct auditory nerve responses were detected at the end of the surgery. In the second case, despite the positive direct responses, the evoked potentials were already not elicited.
In the third patient, hearing was not preserved despite elicited evoked responses and direct responses at the end of the procedure. In the fourth patient with a Koose size IV tumor and non-elicited evoked responses from the beginning of the procedure, a direct response from the auditory nerve was detected.
The auditory nerve then had to be discontinued due to its infiltration by the tumor. Conclusion: Direct monitoring of the cochlear nerve provides the surgeon with real-time information about the state of hearing during vestibular schwannoma surgery.
This method could also be more sensitive in predicting postoperative hearing preservation compared to evoked potentials. Simultaneously with the use of a monitoring probe, it makes it possible to trace the auditory nerve even in the case of larger tumors, in which the success rate in hearing preservation is still very low.